1. Field of the Invention
The present invention relates to a motor start examining device and method for examining, when the rotational speed of a motor is predetermined, a state from the time when the rotation of the motor is started to the time when the rotational speed of the motor reaches the predetermined rotational speed. More specifically, the present invention relates to a start examining device and method of a motor for driving a Polygonal mirror which is provided for an image forming apparatus or the like.
2. DESCRIPTION OF THE RELATED ART
In an image forming apparatus of a laser recording type such as a laser beam printer, a facsimile of a laser recording type and a digital copying machine of a laser recording type, an electrostatic latent image corresponding to an image to be formed is written into a photoreceptor by exposing and scanning the surface of the photoreceptor by a laser beam. The electrostatic latent image on the surface of the photoreceptor is developed into a toner image. The toner image is transferred onto paper sheets and is further fixed thereto, thereby to form an image.
A laser scanner for scanning the surface of the photoreceptor by the laser beam comprises a laser light source, a Polygonal mirror for reflecting the laser beam irradiated from the laser light source toward the photoreceptor, and a motor for rotating the Polygonal mirror at constant speed. The Polygonal mirror is rotated at constant speed so that a light path of the laser beam is changed, to expose and scan the photoreceptor.
In order to accurately write the image by the laser beam, the rotation of the motor must be rigidly controlled. That is, constant-speed rotation control of the motor for rotating the Polygonal mirror is carried out.
A control circuit for realizing the constant-speed rotation control of the motor comprises a stability detecting circuit for determining whether or not the motor is stabilized at a target rotational speed. The stability detecting circuit outputs a rotation stability signal when the rotational speed of the motor is the target rotational speed or a rotational speed within a predetermined allowable range centered around the target rotational speed (hereinafter also referred to as "target rotational speed").
At the time of starting the motor, power is supplied to the motor, the rotational speed of which is quickly raised to the target rotational speed. However, it is actually impossible to monotonously increase the rotational speed of the motor so that the motor is stabilized at the target rotational speed. Specifically, overshoot occurs before the motor is stabilized at the target rotational speed. Consequently, the rotation stability signal is intermittently outputted in a certain time period immediately after starting the motor. That is, the rotation stability signal chatters.
When the rotation stability signal chatters, the rotation of the motor is not-stabilized, so that the Polygonal mirror is not rotated at constant speed. Accordingly, laser light reflected from the Polygonal mirror in this state is distorted.
In the image forming apparatus of a laser recording type, therefore, the laser light source is caused to light up to start an image forming operation after an elapse of a waiting time since the motor for driving the Polygonal mirror was started.
In many cases the waiting time is set to a relatively longer time (for example, approximately 10 to 20 seconds) in consideration of the variation in the starting characteristics of each motor.
After an elapse of the waiting time, the image forming operation is quickly started. After the image forming operation is started, it is judged that a failure has occurred if the rotation stability signal indicating that the motor is being stably rotated at the target rotational speed is broken, whereby the image forming operation is stopped. The reason for this is that in a state where the rotation of the motor is not stabilized so that the rotational speed of the Polygonal mirror is changed, an image to be formed is distorted, thereby to make it impossible to reproduce a clear image.
A problem in such a technique is that the time required until the rotation of the motor is stabilized differs from one image forming apparatus to another, thereby to make it difficult to uniformly determine the waiting time.
More specifically, there are variations in characteristics for each control circuit and motor. Accordingly, the time required until the rotation of the motor is stabilized after starting the motor differs greatly from one image forming apparatus to another. In addition, the time required until the rotation of the motor is stabilized may, in some cases, be longer as the motor is used. If such variations in the time required until the rotation of the motor is stabilized for each image forming apparatus is taken into consideration, the waiting time may be set to the longest time. In this case, however, the waiting time required until the laser light source lights up to start image formation since the motor for driving the Polygonal mirror was started becomes significantly long.
On the other hand, if the waiting time is set to a relatively short time, the chattering of the motor may not, in some cases, completely subside even after an elapse of the waiting time. In this case, the image forming operation is started after the elapse of the waiting time. If the rotation stability signal is broken after the image forming operation is started, however, it is judged that a failure has occurred, whereby the image forming operation is interrupted. Particularly even when the rotation stability signal is slightly broken so that the distortion of an image formed, for example, is not a problem, the image forming operation is interrupted.
As still another method of control, it is also considered that the motor has been started earlier in consideration of the rise time of the motor. However, the driving time of the motor may, in some cases, be longer than necessary. The facsimile of a laser recording type is taken as an example. In this example, it is considered that the motor is started immediately after the receiving of an image is started. In the facsimile, however, it may, in some cases, take only several seconds and in the other cases, take 20 minutes to receive an image corresponding to one page. If the motor is started immediately after the receiving is started, therefore, the motor is uselessly rotated and driven. As a result, the life of the motor may be reduced.